Geology and metallogenetic potential of the anorthositic-leucogabbroic Lagoa da Vaca Complex, Curaça, Bahia State, Brazil / Geologia e potencial metalogenetico do complexo anortositico-leucogabroico de Lagoa da Vaca, municipio de Curaça, Bahia

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

The complex is one of the three litho-structural domains mapped (central domain), and has an outcrop width of 3 kilometers. It is made up mostly of north-south-trending layers of anorthositic, meta-leucogabbroic, meta-gabbroic and ultramafic compositions. Ultramafic rocks of unknown stratigraphic relations, vz. peridotite and amphibole meta-peridotite, were also included in the complexo MineralogicalIy, the anorthositic rocks contain hornblende and labradorític plagioclase, whereas the peridotitcs contain olivine, pyroxenes, spinels and amphiboles. Primary structures are given by mineral layering and syn magmatic slump fauJts. The limit ofthe complex to the North is unknown. 1t is bounded, to the East, by granulite facies rocks of enderbitic to jotunitic composition (eastern domain), and to the West by quartz-feldspatic gneisses with interleaved bands of amphibolite (western domain) which have been strongly deformed by the Caldeirão beJt orogenic event.The tectonic and metamorphic evolution ofthe mapped arca is described in terms ofthree deformation phases; the first two and the last one have respectively ductile and brittle characteristics. Granulite facies metamorphic conditions have been attained during the first phase, folIowed by retrogression (vz. decompression textures, hydration) under amphibolite facies conditions, which is a characteristic ofthe second deformation phase. The third phase, on the other hand, is locally observed in alI domains; ít shows typical brittle-ductile characteristics, and metamorphic mineral assemblages diagnostic of the greenschist facies. The Lagoa da Vaca anorthositic rocks were dated by the whole-rock Pb-Pb isochron technique, yielding an age of 3, 161:J: 65 Ma. Zircons from a granulite have yielded an age of 3072:1:72 Ma by stepwise Pb evaporation technique. Pyroxenes trom the Lagoa da Vaca Complex and peridotites show a significant compositional gap, suggesting that the two rock types most Jikely are not consanguineous nor cogenetic. The lack of sulphide or oxide concentrations, combined with spinel compositions, indicate that both anorthositic and peridotitic rocks have little potential for Chromium, Copper and Nickel mineralisations. The Lagoa da Vaca Complex is very much like other Archaean layered anorthositic complexes found in high grade terranes, such as the Fiskenaesset (Greenland) and Messina (South Africa). Its age, Pb isotopic characteristic (", = 8,8:1: 0,62) and field relationships strongly suggest a continental environment for its emplacement rather than an oceanic one

ASSUNTO(S)

gabro - bahia anortosita - bahia geologia - bahia tempo geologico metalogenia - bahia

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